The present invention concerns, generally, tractors for feeding or advancing web material such as edge perforated webs of paper. As such, the devices of the present invention are especially suited to drive webs of paper such as those employed in computer printout equipment, teletypewriters, optical character readers and the like. However, the tractors capable of broader application, in general for feeding a web of paper or paper-like material in either continuous web or individual sheet form.
Devices adapted to this purpose are, of course, old and well-known in the art. Generally, such tractors include an endless belt trained in a loop over sprocket wheels which are driven by a drive shaft of the equipment (printer, etc.) on which the tractor is mounted. The belt typically has pins projecting from it to engage perforations provided in the paper or other material to drive the material along a linear path. The belt is trained over supports to curve away from the linear path and then return along the loop back to the linear path. Such tractors are normally employed in pairs at opposite edges of the web of material to drive it from both its opposite edges.
One problem associated with such prior art devices is that of maintaining proper transverse alignment of the pins of the spaced-apart paired tractors. The web material has a line of perforations adjacent its two opposite edges and the perforations are transversely aligned relative to the direction of travel of the web material. That is, each perforation adjacent one edge has a corresponding perforation adjacent the opposite edge, both perforations lying on a common line perpendicular to the direction of web travel. Since the pins of the two tractors thus must be aligned transversely across the space through which the web travels, care must be taken when assembling the tractors to effectuate such alignment, which is normally done manually, because the tractors are normally mounted on a common drive shaft. Thus, if the pins of the paired tractors are not transversely aligned once they are assembled, the alignment cannot be corrected simply by rotating one tractor drive while holding the other since both will rotate simultaneously and it becomes necessary to remove one of the belts or otherwise disassemble the device to correct the cross alignment.
It is accordingly an object of the present invention to overcome the foregoing and other shortcomings of the prior art by providing a novel tractor construction and a novel assembly for driving web material including a pair of such drive tractors.
It is another object of the present invention to provide a novel drive tractor construction in which the pins of paired tractors are automatically aligned transversely across the web material travel space between them upon assembly of the tractors to a common drive shaft or to synchronized drive shafts. Other objects and advantages of the present invention will become apparent from the following description thereof.